1. Field of the Invention
The present invention generally relates to positron emission tomography, in particular the arrangement of photomultiplier tubes and scintillation blocks for detection of scintillation events for improved image reconstruction.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected is processed to calculate the position of origin of the emitted photon from the target source. The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
One major technique is Positron Emission Tomography (PET), or coincidence imaging. PET devices are generally based on the “Anger” type camera as disclosed in U.S. Pat. No. 3,011,057. Such cameras are comprised of a scintillation crystal, a light guide and an array of photomultiplier tubes. Radiation interacts with the scintillation crystal, which in turn transforms the energy of absorbed quanta into scintillation light photons. The scintillation light photons spread out from the point of emission. A light guide, generally made up of glass, then directs the light photons to a surface of the photomultiplier tubes (PMTs). The PMTs detect the light and produce an output electronic signal that is proportional to the light energy absorbed. The position of the interaction in the crystal is determined by processing the signals of several PMTs around the site of the emission contact.
In PET, two detectors are positioned on opposite sides of the object of interest, and detect simultaneous pairs of gamma photons emitted from the annihilation of a positron of a high energy radiopharmaceutical. The point of interaction on each detector is calculated and a line is drawn between the two points, thereby forming a line of interaction along which the positron annihilation is considered to have occurred.
A PET system can be designed where a number of panel detectors surround a patient volume in a semi-cylindrical pattern. This pattern could, for example, be hexagonal or octagonal. Coincidences are recorded between the panel detectors in such a way that the activity concentration within the patient volume can be reconstructed providing a 3D volume image.
When a scintillation event occurs, light will be distributed to multiple PMTs around a given locus. The PMT where the scintillation occurs will receive the most light, and therefore produce a signal with greater amplitude. The surrounding PMTs, being further removed from the scintillation event site, will detect less light energy and therefore produce a lesser amplitude signal. The signals, along with the PMTs placement in an x-y coordinate system, are used to calculate the situs of gamma interaction with the camera. The information therein is used to reconstruct the image of the target subject.
Due to the natural physical makeup of the PMTs and the arrangement of the scintillation block, the situs of a scintillation event is determined by such indirect means. Therefore improvements in precisely determining the location of scintillation event lead to improvements in resolution of the resulting image. What is needed therefore is a system or method for arranging the components of a panel such that the position of a scintillation event can be more precisely determined.